The present invention is related to a method and an apparatus for designing a printed-circuit board, more particularly, to a method and an apparatus for designing a printed-circuit board in consideration of a temperature distribution in reflowing.
In design of printed-circuit boards (also referred to as printed boards or simply as boards), importance is put on functions and computing performance in operation, and electronic components (referred to simply as components hereinafter) are mounted in high density. Also, in order to ensure the performance after printed-circuit boards are completed, are carried out transmission line analysis for evaluating delay, reflection, cross-talk noise and the like of electric signals caused by wiring patterns and thermal analysis for evaluating thermal problems of the printed-circuit boards caused by heat generation by mounted components. The results of these analyses are considered for designing the printed-circuit boards.
Of these analyses, are known, as techniques of reflecting the results of thermal analysis of printed-circuit boards for arrangement of the components, a component arrangement system in which components are arranged on a printed-circuit board in consideration of heat generation conditions and heat resistance conditions of the individual components, as disclosed by Japanese Unexamined Patent Publication No. HEI 5(1993)-274391; an interactive arrangement system in which components are interactively arranged in consideration of heating values per region of a printed-circuit board as disclosed by Japanese Unexamined Patent Publication No. HEI 6(1994)-290234; and an automatic component arrangement device in which components are arranged in consideration of cooling of heat-generating components as disclosed by Japanese Unexamined Patent Publication No. HEI 6(1994)-301746.
However, these designing methods of printed-circuit boards are all intended to prevent components from being damaged by heat generated when the circuits on the printed-circuit boards operates, but no account is taken of problems during production steps when the components are placed and mounted on the boards.
In current manufacture of printed-circuit boards, surface mounting is a mainstream, and components are mounted (soldered) onto a board in a heating furnace referred to as a reflow furnace. At this time, on the printed-circuit board, some regions are easily heated and some regions are not easily heated because of difference in heat capacity. If components having large thermal capacities are densely mounted on the printed-circuit board, the temperature of joints to be soldered around these components is not easily raised in a reflow-soldering process. If these joints are heated sufficiently, the temperature of components with small thermal capacities are overheated and thermally damaged. Attempts have been made to solve these problems by adjusting heating conditions in a reflow furnace in production process, but the problems are not solved basically. These thermal problems in the production process of printed-circuit boards not only cause difficulty in manufacture of the printed-circuit boards but also are problems directly connected to the reliability of the manufactured printed-circuit boards.
Further, in recent years, from the viewpoint of the protection of the environment, lead-free solders are tried and attention is paid to a lead-free solder of an amalgam of silver and tin which is highly reliable. The solder of an amalgam of silver and tin has a higher melting point of about 210xc2x0 C. than a tin-lead solder (having a melting point 183xc2x0 C.). Accordingly, it becomes more important to heat the printed-circuit boards uniformly at reflowing.
However, in conventional designing of the printed-circuit boards, importance is attached only to their performance after their completion, and consideration is hardly given to uniform heating of the printed-circuit boards at reflow-heating.
In view of these respects, the present invention is to provide a method and an apparatus for designing a printed-circuit board which allow for uniform heating of the printed-circuit board at reflow soldering.
The present invention provides a method for designing a printed-circuit board comprising the steps of: inputting beforehand data necessary for designing and thermally analyzing a printed-circuit board on which a component having a solder joint is mounted; thermally analyzing the printed-circuit board by simulating the heating of the printed-circuit board in a reflow furnace with use of the input data necessary for the thermal analysis; evaluating from a result of the thermal analysis whether or not there exists at least either one of an unmelted solder joint whose peak temperature does not reach a predetermined lower limit temperature at heating and an over-heated component whose temperature exceeds a predetermined upper limit temperature at heating; according to a result of the above evaluation, changing a design of a component having the unmelted solder joint and/or the over-heated component so that the temperature of the unmelted solder joint and/or the over-heated component is within a range from the lower limit temperature to the upper limit temperature; and outputting the result of change of the design.
According to the present invention, in the designing of a printed-circuit board, consideration is given to the uniform heating of the printed-circuit board at reflow-heating. Thus, it is possible to produce a reliable printed-circuit board having stably soldered joints which is free of damaged components.